A film scanner that passes film across a linear scanning gate must maintain the film absolutely in a flat plane along the length of the scanning aperture and must also advance the film through the scanning zone at a constant scan pitch in order to achieve proper scanning of the image on the film. It is known to provide an arced film scanning surface so that the scan direction of the film lies in a flat plane as the film passes over the scanning aperture. This procedure requires that the film be held in slight tension as it passes over the scanning aperture. Additionally, the rate of advance of the film across the scan gate must be held constant in order that the resultant lines of image scan be uniformly spaced in the direction of film motion across the scanning aperture. This tensioning and uniform rate of film drive can be achieved without great difficulty using a stepper motor to drive a single nip roller set downstream to pull the film from the film supply reel. In a scanner designed to pull film from an Advanced Photo System film storage cartridge, it has been found that the normal retention force of the film spool is somewhat irregular which adversely affects the uniformity of the film feed rate across the scanning aperture. For this reason, it is desirable to utilize a mechanism, such as an upstream nip roller set, to isolate the film drive from the irregular retention forces of the film cartridge. The introduction of an upstream nip roller set, however, introduces additional problems in holding the film feed rate constant across the scanning aperture.
U.S. Pat. No. 5,261,754 discloses a paper feeding device for a printer that utilizes a timing belt, i.e. impliedly an inelastic belt, to drive upstream and downstream nip roller sets at the same angular velocity and avoids irregular paper feed across a linear print head by tensioning the paper between the upstream and downstream roller sets. The tensioning is accomplished according to the explanation in the patent by employing metal, i.e. non-compliant, drive rollers and compliant pinch rollers and by applying a higher degree of pressure on the downstream pinch rollers than on the upstream pinch rollers. According to the explanation, the higher pressure on the downstream pinch rollers deforms the pinch roller surface engaging the print side of the paper stock causing the paper feed amount through the downstream nip roller set to be slightly greater than the paper feed amount through the upstream nip roller set thereby maintaining tension in the paper and avoiding irregular paper feed through the print zone.